1. Field of the Invention
The present invention is generally directed to the field of air bag deployment systems. More specifically, the present invention is directed to an automatic air bag deployment system which includes circuitry for preventing an accidental triggering of the air bag deployment system upon the occurrence of an excessive external magnetic field.
2. Description of the Related Art
Air bag deployment systems are generally very well known and have achieved tremendous commercial success in the automobile industry. Air bags for the front passengers in an automobile are standard equipment on most cars throughout the world and many cars are now equipped with side impact air bags as well. More recently, air bag protection systems have been considered for use in transport aircraft.
Conventional air bag deployment systems may include one or more Hall effect magnetic sensors which are used in accelerometers to determine when an air bag should be deployed in order to prevent injury to occupants during a collision. The accelerometers are used to detect extremely rapid acceleration which is associated with a collision. When such an event is detected, the system then deploys the air bag before the occupants impact the interior surfaces of the automobile.
There are a wide variety of ways in which an accelerometer system can be implemented. Conventional systems may employ Hall effect magnetic sensors of some type. In such a system, the Hall effect sensor senses a variation in a magnetic field which, due to the known arrangement of the Hall effect sensor with respect to other structures, signifies a rapid acceleration of the system. When the system detects such an acceleration, it generates an electric signal which is used to trigger deployment of one or more air bags.
One problem with these existing systems is that there is no provision for preventing accidental deployment of the air bag upon the occurrence of an excessive external magnetic field. This is true notwithstanding the fact that air bag deployment systems are subjected to a wide variety of external magnetic fields from a large number of different sources. The wide variety of magnetic fields that are present in the typical environment in which air bag systems are employed include magnetic fields generated by electric power transmission lines, the magnetic field of the earth, as well as numerous other sources.
Although the stronger magnetic fields that are more likely candidates for accidentally triggering a Hall effect accelerometer are generated by electrical currents, the sum effect of the fields from all other magnetic field sources could also trigger such an event. Because the air bag deployment systems rely upon Hall effect sensors, it is possible that one or more magnetic fields which are not associated with an impact event could inadvertently trigger an automatic air bag deployment system.
Injury could occur as a result of inadvertent deployment. The deployment of an air bag due to the fact that the deployment of an air bag is a very rapid event that requires extremely fast expansion of the bag in order to cushion the impact between the occupants and the internal surfaces of the passenger compartment. Therefore it is apparent that inadvertent deployment of the bag should be avoided.
In order to prevent inadvertent deployment of the air bags, the Hall effect sensors are conventionally located within a magnetic field shielding structure or housing. However, because the magnetic shield material is not impervious to magnetic fields, though unlikely, it is still possible that the system could be inadvertently deployed if the system were to encounter a magnetic field of sufficient magnitude. There is currently no known system for preventing an accidental deployment of an air bag upon the occurrence of an excessive external magnetic field. As detailed below, the inventors of the present application have overcome these shortcomings of the prior art. Other objects and advantages of the present invention will be apparent in light of the following Summary and Detailed Description of the Preferred Embodiments.
The present invention is directed to an air bag deployment system which includes circuitry for preventing the accidental deployment of the air bag during exposure of the system to an excessive external magnetic field. In a first preferred embodiment of the system, the system includes an accelerometer comprised of first and second Hall effect magnetic devices which are associated with the impact detection circuitry of the deployment system. These are the sensors which would normally trigger deployment of the air bag system upon detection of an acceleration of sufficient magnitude. The sensors are conventionally located within a magnetic field shielding structure or housing in order to limit the potential for accidental triggering of the automatic deployment system,
During typical operation, these accelerometers sense motion of an object which alters a magnetic field. The sensed magnetic field is thus used in determining the presence of an acceleration which is of sufficient magnitude to require deployment of the air bag in order to prevent injury to the occupants of a vehicle.
The advanced system of the present invention further includes circuitry which senses external magnetic fields. This sensing circuitry may be comprised of one or more additional Hall effect magnetic sensors which are located preferably outside the magnetic shield that contains the primary circuitry for triggering deployment of the air bag. These additional sensors may also be located within the magnetic shield. The preferred external location of the Hall effect magnetic sensors ensures that these sensors will accurately detect the presence of any significant external magnetic field.
This is particularly important when the external magnetic field may be sufficiently high to result in an accidental triggering of the air bag deployment mechanism. Although it is understood that a single Hall effect magnetic sensor may be employed for sensing the presence of an excessive external magnetic field, it is desirable that at least two of the sensors be used at different orientations to ensure that the system will accurately detect the presence of an undesired external magnetic field having any orientation. These additional sensors provide an output with a sufficiently high threshold level to ensure that those magnetic fields which typically exist are not of the sufficient magnitude to result in disabling of the air bag deployment system. Furthermore, once an excessive external magnetic field has been detected with this circuitry, the system will not be permanently disabled. (It will be totally disabled only for a finite time. It will reset itself after a brief period of time in order to ensure that the system is working should an actual collision take place.
A variety of different circuits may be employed for disabling the air bag triggering circuit when such an excessive magnetic field is encountered. In a first preferred embodiment, the circuitry is comprised of logic gates arranged to appropriately disable the air bag deployment triggering mechanism upon sensing an excessive external magnetic field. For example, the system may employ an AND gate which receives a first input from the output of the triggering circuitry and a second input which, depending upon convention, will either be the inverted or non-inverted output from the external magnetic field sensing circuitry. This arrangement will ensure that the ultimate output from the AND gate will change states only when a sufficiently high acceleration has been sensed without the presence of an excessively high external magnetic field.
In an alternate embodiment, rather than logic circuitry, a simple switch may be used to provide an open circuit from the output of the air bag deployment triggering circuit. The switch is used to cause an open circuit condition on the signal line which travels from the air bag deployment trigger signal generating circuit to the actual air bag deployment mechanism. Depending upon convention, the output from the switch will be held either at a low condition or a high condition through a resistor which is either connected to ground or a High logic level voltage as is generally known in the art in order to ensure that the signal line does not inadvertently float to a level which would result in deployment of the air bag. The external magnetic field sensing circuitry is connected to the switch to open the switch when an excessive magnetic field is present.
In yet another alternate embodiment of the present invention, the excessive external magnetic field sensing circuitry provides an interrupt signal to a microprocessor which controls the actual air bag deployment system. Upon receipt of this interrupt signal, the air bag deployment signal system will become disabled for a predetermined period of time. This delay may be programmed into the system and may be virtually any fixed length period of time.